1. Field of the Invention
The invention pertains to the field of microwave attenuators and more particularly to matched microwave variable attenuation in monolithic microwave integrated circuits.
2. Description of the Prior Art
Microwave attenuators of the prior art provide attenuation in either discrete step attenuation or continuously variable. The continuously variable attenuators have one of two classic configurations; the resistive Pi circuit and the resistive T circuit. These attenuators utilize either PIN diodes or field effect transistors (FETs) for the series and shunt resistors. PIN diodes and FETs exhibit resistive changes with properly applied DC voltages and thus are useful as variable resistors. The resistive values of these Pi and T circuits for all levels of attenuation are chosen to provide an impedance that matches the impedance of a transmission line, or another microwave device, to suppress reflections in the system. To accomplish this, the ratio of the shunt and series resistors must change with attenuation changes, establishing a functional relationship of the ratio versus attenuation which is extremely non-linear. This presents a very difficult tracking problem, requiring that the dc characteristics of the PIN diodes or FETs utilized in the attenuators be matched over the entire attenuation range. As a result, the PIN diodes and FETs are generally controlled with separate power supplies. Though control circuitry can be provided to supply the DC voltages to the voltage controlled resistances in their proper functional relationship from a single power supply, such circuitry requires much more real estate than the attenuator it controls and is therefore rejected for most applications.
The problem of providing the proper ratios to maintain a constant characteristic impedance for the Pi and T circuits is exacerbated by the non-uniformity of the PIN diode and the FET characteristics that result with present day manufacturing processes. For example, the equivalent resistance value of a FET is a function of the pinch-off voltage, that voltage which must be exceeded by the gate voltage for current to flow in the FET. Present day manufacturing processes, however, yield FETs with pinch-off voltages that vary substantially. Since the resistance of the FET is a function of the pinch-off voltage, FETs exhibit resistance values having varying functional relationships of the gate voltage. Thus, for each attenuator a process is encountered for selecting three FETs with equal resistance versus gate voltage characteristics, greatly increasing cost of the attenuators.
Further, resistive Pi and T circuits cannot simultaneously realize low (off) state insertion loss and a large dynamic attenuation range with the variable resistors presently available. For both circuits a low insertion loss requires a low resistance value for the series elements and a high resistance value for the shunt elements. As attenuation increases from the minimum value the series resistance increases, while the shunt resistance decreases. Since the shunt resistances start at opposite ends of the functionality curve it is extremely difficult to provide the ratio of series resistance to shunt resistance required for the attenuation values desired and simultaneously maintain a constant characteristic impedance for the circuits.
Additionally, at high frequencies, the internal capacitances of the PIN diodes and FETs establish complex characteristics for the Pi and T circuits. To provide real characteristic impedances it is necessary to resonate these capacitances by shunting inductors across the elements of the Pi and T circuits. These resonant circuits severely limit the operating bandwidth of the attenuator.
An attenuator which provides improved performance over the Pi and T circuits is disclosed in U.S. Pat. No. 4,970,478 issued to the assignee of the present invention. This patent discloses a variable microwave attenuator which includes a plurality of ladder circuits (cells) each having a series inductance and a shunt circuit comprising a capacitor and a variable resistor in parallel. The cells are cascaded in a manner to establish an artificial transmission line with distributed loss provided by the variable resistor shunt elements. These variable resistor shunt elements may be realized by utilizing FETs which exhibit resistive changes of voltage applied to their gates. The series inductance, shunt capacitance, and shunt variable resistors are chosen to establish an impedance for the artificial line that is substantially independent of the shunt resistance value and to provide a low reflection coefficient with its concomitant low voltage standing wave ratio (VSWR). When cascaded, the internal ladder sections combine to form lossless symmetrical Pi cells with a variable resistor positioned between each cell. Symmetry of the artificial line may be completed with the addition of a shunt capacitor at one end of the artificial line to establish lossless symmetrical Pi end sections for the transmission that are identical to the internal lossless symmetrical Pi sections formed by cascading the ladder networks.
Though the artificial line of U.S. Pat. No. 4,970,478 provides variable attenuation by the adjustment of but one resistance value per cell and may provide a characteristic impedance which is independent of the shunt resistance value, such performance is difficult to achieve. Further, due to the resistance variation of PIN and FETs, previously described, a variable attenuator which provides attenuation with reasonable precision requires extensive calibration, adding appreciable cost to the device.
Another prior art attenuator which minimizes the problem of maintaining the characteristic impedance with attenuation variations is disclosed in U.S. Pat. No. 5,109,204 entitled "High Power RF Precision Attenuator" issued to Lyndon M. Keefer on Apr. 28, 1992 and assigned to the assignee of the present invention. This patent discloses a variable microwave attenuator utilizing a first pair of isolated quadrature hybrid ports as the input and output ports of the attenuator and providing the second pair of isolated ports with switchably coupled resistive terminations, the resistance at each termination being equal and selected in accordance with the attenuation desired. Since these resistors are not matched to the hybrid's characteristic impedance the portion of the signals incident to the terminated ports, that are not dissipated in the resistive termination, are reflected and coupled in the hybrid in a manner to be out-of-phase at the input port, thus cancelling thereat, and to be in-phase at the output port, thus adding thereat to provide the attenuated output signal. A throughput without attenuation is provide by shunting diodes across the terminating resistors. When these diodes are in the conducting state, short circuits are established which reflect the entire signal to the output port. This arrangement is wasteful of energy, requiring dc power whether or not the device is attenuating microwave signals. Further, conducting diodes do not provide complete short circuits. Thus some attenuation is realized in the throughput state.
Another variable attenuator of the prior art provides attenuation by switchably coupling resistors of equal value across the two output ports of a quadrature hybrid circuit, the output ports are then coupled to the input ports of a second quadrature hybrid. One input port of the first hybrid and one output of the second hybrid are terminated with a resistor having a resistance equal to the characteristic impedance of the hybrid to absorb power coupled to these ports. The remaining input port of the first hybrid and the remaining output port of the second hybrid serve as the input and output ports of the attenuator, respectively. Attenuation control is accomplished with switching diodes in series with the attenuator resistors. Each of these diodes are independently controlled through respective driver circuits directly connected to the associated diode. The attenuator therefore requires an appreciable number of circuit elements for the driver assembly and appreciable switching power to effectuate an attenuation change.